1. Field of the Invention
The present invention relates to a color filter having a light-transmitting colored part, a process for producing the color filter, and a liquid crystal panel employing the color filter.
2. Related Background Art
With the recent development of personal computers, especially portable personal computers, the demand for liquid crystal displays, in particular for liquid crystal color displays, has been increasing. In order to meet the increasing demand for liquid crystal color displays, a reduction in the cost thereof is necessary, especially a reduction with respect to the color filters which substantially comprise the cost. Although various methods have been proposed to meet the above needs while maintaining the properties necessary for color filters, there are still unsolved problems at present. For example, many, proposals have been made for color filter manufacturing to use an ink-jet system having a potential of effective cost reduction. When the ink-jet system is used for color filter production, however, it is often difficult to achieve both stable ejection performance of the ink and a high standard of product reliability such as water resistance and light fastness of the color filter. In order to produce a light-transmitting colored part by the ink-jet method, a composition containing a coloring material such as a dye or a pigment in a mostly water aqueous solvent is used as the ink. When a dye is used as a coloring material, ejection stability of the ink in the ink-jet system is satisfactory to a certain extent, but the product reliability of the color filter thus produced is generally inferior to those produced using a pigment coloring material. On the other hand, when a pigment is used as a coloring material, the product reliability of the color filter is satisfactory to a certain extent, but the ejection stability of the ink in the ink-jet system is generally inferior to those using a dye coloring material. Further, there is a problem that pigment-type water-based inks are sometimes inferior in storage stability over a long period of time.
On such a background, studies have been made of ink-jet systems using water-based pigment inks which can achieve both ink-jet ejection stability and long storage stability. This is explained in detail below.
It is most preferable to use a pigment as the coloring material when the product reliability of the color filter is considered. When a pigment which is conventionally used for the color filters is used as a coloring material in an ink-jet water-based ink, it is necessary to disperse the pigment in the medium in a stable condition. In general, the pigment is dispersed in an aqueous medium by adding a dispersant to form a homogeneous dispersion system. However, there is a problem that sometimes the dispersion state is not satisfactory even if a dispersant is used, and thus the ink containing a dispersed pigment is inferior in long time storage stability.
Meanwhile, when an ink is used in an ink-jet system, the ink should be ejected from the tip of a minute nozzle (orifice) of the ink-jet recording head as a stable droplet. Thus, it is necessary to prevent solidification of the ink by drying at the orifice. When an ink containing a dispersant as described above is used in the ink-jet system, the dispersant component such as a resin may stick to the orifice without re-dissolution to cause clogging at the orifice or fault in ejection. There is also a problem with the water-based pigment ink containing a dispersant that it is so viscous that ejection may become unstable when continuous ejection and high speed image formation is carried out for a long time, due to the high resistance in the passage to the nozzle tip.
Japanese Laid-Open Patent Application No. 11-80633 discloses the production of a color filter by using an ink containing fine colored particles of a pigment covered with a film-forming resin having carboxyl groups, the particles being dispersed in an aqueous medium.
The inventors of the present invention, however, have determined that the pixels formed by an ink-jet system using an ink containing a pigment 19 covered with a thermo-crosslinking resin as shown in Japanese Laid-Open Patent Application No. 11-80633 do not have sufficient pixel transparency, which is one of the important properties for color filters. The reason why the pixels do not have enough transparency is believed to be that the pigment is localized in the pixel and such localization causes random reflection. The mechanism for the localization will be explained with FIGS. 1A, 1B and 1C. That is, when a base plate 1 is subjected to at least one of heat treatment and light irradiation after the attachment of the ink (FIG. 1A), cross-linking between the resins covering the pigment 19 and evaporation of the aqueous medium will occur (FIG. 1B). Thus, pigment particles tend to draw together and localize in the pixels (FIG. 1C) to cause random reflection at the regions of the pigment localization to reduce transparency. Based on such a finding, the inventors of the present invention have decided to develop a pigment ink which has properties highly suitable for an ink-jet system, which can provide pixels of high transparency, and which can provide color filters of required properties.
An object of the present invention is to provide a color filter and a production method thereof which utilizes a water-based pigment ink excellent in both long storage stability and ejection stability in an ink-jet system, which ink-jet system can effectively reduce production cost.
Another object of the present invention is to provide a liquid crystal panel containing a color filter of high quality, product reliability and low production cost, which color filter has light-transmitting colored parts formed by an ink-jet system using a water-based pigment ink.
According to one aspect of the present invention, there is provided a process for producing a color filter comprising a step of arranging a light-transmitting colored part on a surface of a base plate by applying an ink onto the base plate by an ink-jet system, wherein the ink comprises water, an aqueous organic solvent and a coloring material, and the coloring material contains a self-dispersing pigment having at least one species of hydrophilic group bonded directly or through a linker group to a surface thereof.
According to another aspect of the present invention, there is provided a color filter comprising a self-dispersing pigment at a light-transmitting colored part, wherein the light-transmitting colored part contains a self-dispersing pigment having a hydrophilic group bonded directly or through a linker group onto the surface.
According to still another aspect of the present invention, there is provided a liquid crystal panel comprising a first base plate having a pixel electrode, a second base plate having a common electrode, a liquid crystal composition layer enclosed between the first and second base plates, and a color filter having a light-transmitting colored part arranged corresponding to the pixel electrode, wherein the color filter is the color filter described above.
According to the present invention, a color filter of high quality can be produced at a low cost by advantageously using an ink-jet system with a pigment-type coloring material having excellent stability in long storage as well as stable ejection properties when used in an ink-jet system. Obtainable is a color filter having a light-transmitting colored part of high transparency, and the filter also has highly reliable properties such as water resistance and light-fastness.
FIGS. 2A, 2B and 2C schematically illustrate the formation process of the colored part of the color filter of the invention. An ink is applied to a base plate 1; a self-dispersing pigment 20 is uniformly dispersed in the ink (FIG. 2A). At least either of heat treatment and light irradiation causes cross-linking of the binder component in the ink and evaporation of the medium of the agent (FIG. 2B). As a result, the self-dispersing pigment 20 distributes uniformly between the cross-linked binder component (FIG. 2C), and the resultant film has a smooth surface to provide good transparency.
There have been already known self-dispersing pigments formed by bonding a hydrophilic group directly or through a linker group to the surface of the pigment, or inks using such pigments, as disclosed in, for example, U.S. Pat. No. 5,837,045. However, nothing has been disclosed about the properties of the pixels formed by an ink-jet system using such an ink in the color filter production, and it has not even been suggested that such an ink can form pixels having properties sufficient to be used in a color filter.
FIGS. 1A, 1B and 1C illustrate a prior art production process of pixels of a color filter using an ink containing pigment particles covered with a film-forming resin.
FIGS. 2A, 2B and 2C illustrate a formation process of a colored part of a color filter using an ink used in the present invention.
FIGS. 3A, 3B, 3C and 3D illustrate a production process of a color filter according to the method of the present invention.
FIGS. 4A, 4B, 4C, 4D and 4E illustrate another production process of a color filter according to the method of the present invention.
FIG. 5 illustrates an example of a liquid crystal panel using a color filter obtained by the method of the present invention.